Multiple-input and multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) technologies are becoming increasing popular for wireless data communication networks. It is well-known that the spatial degrees of freedom, which MIMO techniques provide, increase data rates without the need for excess transmission bandwidth. OFDM also provides frequency-selective scheduling gains in a MIMO cellular network. Furthermore, the combination of MIMO with OFDM modulation provides high data rate capabilities over wider transmission bandwidths with improved reliability against time- and frequency-selective channel fading, multi-user diversity and interference in cellular wireless networks.
By varying the modulation format, channel code rate, transmission power and signaling duration, adaptive data transmission can exploit the variation of wireless channels to improve reliability performance and increase data throughput. Because MIMO-OFDM wireless networks are characterized by channel selectivity in space, time and frequency domains, conventional wireless networks, such as networks designed according to the IEEE 802.11n, IEEE 802.16e and 3GPP LTE standards, use channel sounding by multiplexing known pilot symbols (pilot tones) with unknown data symbols in an OFDM symbol, i.e., the pilot symbols and the modulation data symbols do not overlap. Thus, a receiver can estimate the space-time-frequency channel and feed back a quality metric for the channels to a transmitter. Then, the transmitter can adjust its transmission parameters, such as modulation format, channel code rate, transmission power and signaling duration to adapt the transmissions to the channels in a rapidly varying environment.
Unfortunately, multiplexing of known pilot symbols with unknown data symbols leads to a reduced number of the data symbols within each OFDM symbol, thereby significantly reducing the data rate per OFDM symbol. More importantly, because the number of pilot symbols needed to sound a highly frequency-selective fading channel is different from the number of pilot symbols needed to sound a frequency-flat fading channel, a priori multiplexing of pilot symbols and data symbols at a specific overhead either wastes the number of sub-carriers to sound a frequency-flat fading channel, and the overhead is insufficient to sound a highly frequency-selective fading channel. Also, because many different configurations are possible in multiplexing the pilot symbols and the data symbols within the sub-carriers of the OFDM network, the consequence is that design and implementation of conventional channel sounding (CS) methods are less flexible.
Implicit Pilot Symbols
In our related application Ser. No. 12/827,591, we first describe the basic principals of our implicitly embedding of pilot symbols in data symbols of resource blocks in MIMO-OFDM networks. That Application deals with the relatively simple case of an open-loop network where there is no feed back of channel state information from the receiver to transmitter. There the assumptions are that the channel coherence time is relatively long, e.g., for an entire resource block or more, and the channel coherence bandwidth is relatively constant. This is adequate for many networks wherein the configuration of the network and the environment is relatively static, e.g., indoor networks where the receivers in computers, laptops and wireless telephones generally move infrequently. In such networks, it is not important to adapt modulation and coding schemes to a rapidly varying environment.
Now, we want to deal with the more difficult case where the above assumptions are not true, and the environment does vary, as in networks operating outdoors, and where the transceivers are highly mobile.